Surface Polarization in Al₂O₃‑Capped GaN/AlGaN/GaN Heterostructures Investigated via Angle‑Resolved X‑ray Photoelectron Spectroscopy

Background

GaN’s high breakdown field, electron mobility, and thermal stability make it a leading material for LEDs and power electronics [1,2]. In GaN high‑electron‑mobility transistors (HEMTs), a Schottky gate introduces interface states that degrade leakage current and reduce the breakdown field [3]. Passivating the surface with a high‑k dielectric such as Al₂O₃ mitigates these issues due to its wide band gap, high dielectric constant, and thermodynamic stability [4,5]. However, ALD of Al₂O₃ inevitably creates a thin interfacial layer at the GaN surface [6–8]. This layer, often described as GaO_x, strongly influences threshold voltage stability, two‑dimensional electron gas (2DEG) density, and band‑bending behavior [9–12]. While previous studies have identified the existence of the interfacial layer, its detailed role in polarization modulation has not been fully elucidated. Here, we employ ARXPS to quantify band‑bending evolution and to resolve the atomic structure of the interfacial layer in Al₂O₃‑capped GaN/AlGaN/GaN structures.

Methods

The samples consist of a 2‑nm GaN cap, 22‑nm AlGaN channel, and 150‑nm intrinsic GaN buffer on Si(111). Three wafers (S1, S2, S3) were processed identically: cleaned in acetone, isopropyl alcohol, and deionized water; etched with dilute HCl to remove native oxide; then capped with Al₂O₃ by ALD using trimethyl aluminum and H₂O. The Al₂O₃ thickness was 1 nm for S1 and 3 nm for S2 and S3 (verified by ellipsometry). S3 underwent a 5‑min N₂ anneal at 400 °C to promote interfacial oxidation.

ARXPS measurements were carried out on a Thermo Fisher Scientific Theta Probe equipped with a monochromatic Al Kα source (1486.6 eV). Binding‑energy calibration used Ni, Au, Ag, and Cu standards. Spectra were recorded at incidence angles (θ) ranging from 27.5° to 72.5° relative to the sample normal, without tilting the specimen. All spectra were referenced to the C 1s peak at 285.0 eV to correct for charging effects. Quantitative analysis employed the Avantage software’s relative sensitivity factors.

Results and Discussion

Ga 3d core‑level spectra for each sample reveal distinct bonding environments. S1 shows two components (Ga–N and Ga–O), while S2 and S3 exhibit an additional O 2s peak attributable to Ga–O and Al–O bonds, becoming more prominent at higher detection angles.

The binding‑energy shift of the Ga–N peak (ΔBE) with θ provides direct insight into band bending. S1 displays a 0.20 eV decrease from θ = 27.5° to 72.5°, indicative of upward band bending consistent with literature [13]. S2 shows a smaller 0.10 eV decrease, suggesting a flatter band profile. In contrast, S3 exhibits a 0.20 eV increase in BE, revealing downward band bending after N₂ annealing.

Al 2p spectra remain unchanged across all samples, confirming that AlGaN does not influence the Ga–N BE variations. Table 1 (summarised in the article) lists the BE values for Ga 3d and Al 2p at each θ with ±0.1 eV uncertainty.

The Ga–O to Ga–N intensity ratio, calculated from peak areas and sensitivity factors, rises from ~0.2 in S1/S2 to ~0.3 in S3, signalling enhanced GaO_x formation post‑annealing. The Ga/N ratio drops from ~1.7 (Ga‑rich) in S1/S2 to ~1.0 (stoichiometric) in S3, indicating that the interfacial Ga‑rich layer is oxidised into GaO_x.

Considering the electron inelastic mean free path (~3.4 nm for Ga 3d in Al₂O₃), the sampling depth at each θ is 3λ cos θ minus the Al₂O₃ thickness. Thus, higher θ probes shallower depths, accentuating the Ga–O signal and raising the Ga–O/Ga–N ratio. The transition from Ga–N to GaO_x effectively removes the intrinsic negative polarization of GaN, flattening the conduction band and introducing positive interfacial charges that drive the band edge downward.

Annealing further oxidises the Ga‑rich surface, thickening GaO_x and increasing the density of positive charges. These charges create a fixed‑charge layer that modifies the internal electric field, turning upward band bending into downward bending. The observed BE shift with θ in S3 confirms this mechanism.

Importantly, the GaO_x layer raises the interface barrier height (Φ_b), enhancing 2DEG mobility while reducing its density, as reported in prior studies [14,15].

Conclusions

ARXPS measurements demonstrate that Al₂O₃ capping on GaN/AlGaN/GaN heterostructures generates a Ga‑rich interfacial layer and a subsequent GaO_x layer. These layers neutralise the intrinsic GaN polarization, introduce positive fixed charges, and transform the band profile from upward to downward bending at the interface. This insight is critical for engineering high‑performance, stable GaN‑based HEMTs.